Method of condensing magnesium vapor



April 22, 1941. T. H. McCONiCA, so

METHOD OF CONDENSING MAGNESIUM VAPOR Filed Feb. 21, 1940 7 M [0 M/xfurePomp Air/2005;

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roduc/ 4/ (b Mix/are ATTORNEYS Patented Apr. 22, 1941 'METHOD OFCONDENSING MAGNESIUM VAPOR Thomas H. McConica, III, Midland, Mich., as-

signor to The Dow Chemical Company, Midland, Mich., a corporation ofMichigan Application February 21, 1940, Serial No. 320,085

7 Claims.

This invention relates to an improved method of condensing metallicmagnesium from vaporphase admixture with carbon monoxide.

In the preparation of metallic magnesium by the thermal reduction ofmagnesia-containing materials using carbonaceous reducing agents, themagnesium is liberated in the vapor state in admixture with carbonmonoxide and at a. temperature well above that at which any substantialinteraction between magnesium and carbon monoxide occurs; it can berecovered only by condensation. Unfortunately, since magnesium, in bothvapor and liquid states, is vigorously attacked by carbon monoxide atordinary condensing temperatures, it is inevitable that some loss ofmagnesium must occur in any-condensation. To minimize this loss, it iscustomary to subject the vapor mixture to be condensed to drasticchilling so as to condense and then cool the magnesium to temperaturesbelow its freezing point in as short a time as possible.

As methods of effecting shock-condensation and chilling, the art hasrecommended either that large volumes of cold hydrogen or finelydividedsalts be injected into the vapor mixture, or that the mixture be causedto impingeupon extensive cold surfaces. These procedures, althoughoperable, are not wholly successful in preventing appreciable reversionof the liberated magnesium to magnesia, so that frequently as high as40-50 per cent of the oxide is found in the metal condensate.

In view of this situation, it is an object of the present invention toprovide an improved condensing process which permits an even more rapidrate of condensation than that existing in prior processes, and henceproduces a metal condensate containing a considerably lower percentageof magnesia than has been customary heretofore.

According to the invention, the vapor mixture 7 of magnesium and carbonmonoxide to be condensed is first chilled suddenly by injecting acontrolled small proportion of a relatively cool tion or settling.

quence, within a fraction of a second, that they are almostsimultaneous.

The gaseous cooling of the first step may be efiected by any gas whichdoes not oxidize magnesium under the reaction conditions, e. g.,hydrogen, helium, or hydrocarbon vapors. The gas is supplied at such atemperature that it is cool relative to the magnesium vapor;temperatures up to 650 C. are satisfactory, although in practicaloperation the gas is usually not far above room temperature. The gas isinjected into the magnesium-carbon monoxide mixture in a proportionsuflicient to cause a sudden and considerable drop in temperature, butnot sufficient to cool the mixture below the temperature at whichmagnesium condenses at the existing pressure. It is preferable to chillthe vapor mixture just to the point of incipient condensation, althoughit is difficult in practice to achieve temperature control of thisprecision. It is better to inject too little rather than too much gas,since the process of the invention does not contemplate injecting enoughcool gas to cause any substantial condensation of the magnesium.

Immediately after the gas chilling step, the magnesium-carbon monoxidevapor mixture is passed into contact with a non-gaseous condensing meansmaintained at a condensing temperature below 650 C. The condensing meansmay be either liquid or solid; if liquid, it is preferably supplied in aproportion sufficient to absorb the entire heat of condensation of themagnesium. The most suitable liquid condensing agents are hydrocarbonoils, preferably paraflins of high boiling point, from which thecondensed magnesium can be recovered in solid form by filtra- However,molten metals and alloys which dissolve magnesium at temperatures below650 C., e. g. lead, may also be used, the condensed magnesium beingsubsequently recovered from solution by distillation. Themagnesium-carbon'monoxide vapor mixture may be introduced into contactwith the liquid condensing agent in any desired way, for'instance bybubbling it into abody of the latter, or by injecting it into a spray ofthe liquid.

Among the solid condensing means which may be used are ordinary surfacecondensers, made of any suitable material of construction and providedwith adequate coolingmapacity Alternatively, the'chilled vapor mixturemay be con densed by introducing it into a shower of fine metalparticles supplied in a quantity sufllcient to absorb the heat ofcondensation and cooling,

as disclosed in my co-pending application Serial No. 316,628, filedJanuary 31, 1940.

It is important that the non-gaseous condensing means be maintained at atemperature below 650 C. In this way, the condensed metal is cooled atonce to below its freezing temperature; any substantial interaction ofthe condensate with carbon monoxide is avoided and the purity of themagnesium iskept high.

A probable explanation of the improved 'efliciency of the process of theinvention as compared to prior processes is found in theoreticalconsiderations. The condensation of magnesium vapor can be considered asoccurring in four steps: (1) cooling of the magnesium-carbon monoxidemixture from its temperature of formation down to a temperature ofincipient condensation (dew point) (2) condensation to the liquid phase;(3) cooling of the liquid to its freezing point; and (4) freezing of theliquid condensate. Of these stages, the first inyolves abstraction ofheat from a wholly gaseous mixture, Whereas the other three require thatheat be removed. from a liquid, either during or after its formation. Ananalysis of the molecular kinetics involved seems to show that by farthe most effective way of cooling a hot gas is to inject therein a coldgas, whereas except under very unusual conditions a liquid is cooledmuch more readily by contact with a cooler liquid or solid than bycontact with a cold gas. These principles are employed in thepresent-process, which uses gas cooling to chill the gaseousmagnesium-carbon mixture to near its condensing temperature, and thenuses liquid or solid cooling to effect the condensation, cooling, andfreezing of the magnesium. Optimum heat transfer conditions aremaintained at every stage in the process; condensation and cooling isaccordingly accomplished. in a shorter time, and attack of the magnesiumby the carbon monoxide is mini mized.

A preferred form of the process of the invention may be explained indetail with reference to the accompanying drawing, in which:

Figure l is a diagrammatic elevation, in partial section, of onearrangement of apparatus adapted to carry out the invention; and

Figure 2 illustrates another arrangement of apparatus.

The apparatus shown in Figure 1 includes a short vapor supply tube 3which is incommunication at one end with an electric reduction furnace(not shown) or other source of magnesiumcarbon monoxide vapor mixture,and within which is mounted a gas inlet consisting of a hollow ring 4having an inlet nipple 5 on its periphery, and gas ports 6 around itsinner circumference. At its other end, the'vapor tube 3 leads into aspray chamber 1 provided at the top with a spray nozzle 8 and a gasoutlet 9, and at the bottom with a drain III. This drain leads to asettling tank ll having a vent l2 and a bottom outlet 13 fitted with adischarge lock l4. The tank II also has a side outlet l5 leading to acirculating pump I6 which is connected by suitable piping through acooler I! to the nozzle 8 in the spray chamber 1.

In operation, the separating tank II is filled with a hydrocarbon oil;the cooler I1 is turned on, and the pump I6 is set in motion so as toforce cool oil to the nozzle 8, from which it falls recovering thecondensed magnesium.

as a spray through the tower I back to the tank I I. If operation atreduced pressure is desired,

suction is applied to the gas outlets 9 and I2.

I 20 fitted with a discharge lock 2|.

A cool non-oxidizing gas, such as hydrogen is introduced into the vaportube 3, through the I inlet nozzle 4. The magnesium-carbon monoxidevapor is then admitted into the vapor tube 3, where it passes throughthe gas nozzle 4, and is cooled sharply, and then passes immediatelyinto the spray chamber I, where it is quenched by the oil spray andcondensed. The uncondensed carbon monoxide and hydrogen escape throughthe outlet 9. The oil and condensed magnesium run into the separator II,where the metal settles out and may be withdrawn through the lock M; theoil is recirculated through the pump l6 and cooler l'l back to the spraynozzle 8. .As already explained, the rate of gas introduction throughthe nozzle 4 is controlled so that the magnesium-carbon monoxide mixtureis chilled approximately to the temperature of incipient condensation,and the cooler I1 is adjusted to maintain the temperature of the oilbeing recirculated well below 650 C,

An alternative form of apparatus shown in Figure 2 also consists of avapor delivery tube 3 provided with a gas nozzle 4. After the nozzle,the tube 3 leads to a condensing chamber l8 having an upper gas outletI9 and a hopper bottom Mounted within the chamber l8 opposite the vaportube 3 are a rotatable drum 22, which is equipped with adjustablecooling means (not shown), and a scraper 23 placed so that any scrapingsremoved from the drum fall into the hopper 20. In operation, themagnesium-carbon monoxide vapor mixture introduced into the inlet tube 3is subjected to chilling by cool non-oxidizing gas injected through thenozzle 4, and then passes directly into contact with the drum 22, whichis maintained at a temperature below650" C, and is rotated at a slowrate. The magnesium condenses on the drum 22 in solid form and then, asthe drum revolves, is scraped into the hopper 20, from which it may bewithdrawn.

It is to be understood that the foregoing description is illustrativeonly, and that the invention is co-extensive in scope with the followingclaims.

I claim: I

1. The method of condensing magnesium from the vapor phase whichcomprises injecting a relatively cool non-oxidizing gas into the saidvapor in a proportion sufllcient to cause substantial cooling butinsuilicient to chill the vapor to a temperature below that at whichcondensation of magnesium occurs at the existing pressure, immediatelypassing the cooled vapor into contact with non-gaseous condensing meansmaintained at a condensing temperature below 650 (2., and

2. The method of condensing magnesium from a vapor-phase admixture withcarbon monoxide which is at a temperature above that at which anysubstantial reactionbetween magnesium and carbon monoxide occurs, whichcomprises injecting a relatively cool non-oxidizing gas into the saidmixture in a proportion suflicient to cause substantial cooling butinsuflicient to chill the mixture to a temperature below that at whichcondensation of magnesium occurs at the existing pressure, immediatelypassing the cooled vapor mixture into contact with non-gaseouscondensing means maintained at a condensing temperature below 650 0.,and recovering the condensed magnesium.

3. A process according to claim 2 in which the non'-gaseous condensingmeans is 'a'liquid'com" which is at a temperature above that at whichdensing medium supplied in a proportion sulficient to absorb the heat ofcondensation.

4. A process according to claim 2 in which the non-gaseous condensingmeans is a hydrocarbon oil. A

5. A process according to claim 2 in which the non-gaseous condensingmeans is a solid condensing surface.

6. A method according to claim 2 in which the non-gaseous condensingmeans is a shower of metal particles supplied in a quantity sufllcientto absorb the heat of condensation and cooling.

7. The method of condensing magnesium from a vapor-phase admixture withcarbon monoxide,

any substantial reaction between magnesium and carbon monoxide occurs,which comprises injecting a relatively cool non-oxidizing gas into thesaid mixture in a proportion sufiicient to chill the mixtureapproximately to the temperature at which condensation of magnesiumoccurs at the existing pressure, but insuflicient to cause anysubstantial condensation, passing the-chilled vapor mixture into contactwith non-gaseous conden sing means maintained at a condensingtemperature below 650 C., and recovering the condensed magnesium.

THOMAS H. McCONICA, III..

